This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. G protein signaling, the most common mechanism for signal transduction in mammals, involves the formation of multi-protein complexes at the membrane surface. Our focus is on determining the structures of membrane-associated complexes that mediate the G protein cascade of vertebrate vision. Our recent work using cryo-EM has targeted the G protein heterotrimer, transducin, the effector enzyme of vision, cGMP phosphodiesterase, PDE6, and their membrane-bound complexes. Tranducin is an 85 kDa abg heterotrimer which binds photoreceptor membranes through covalently attached lipids: a fatty acid on the a subunit and an isoprenoid on the g subunit. When activated by the G protein coupled receptor rhodopsin, transducin's ? subunit (Gt?) binds GTP and in this form binds to and activates PDE6. The total size of the G protein-effector complex on the membrane surface is likely a 280 kDa heterohexamer containing four different kinds of polypeptides: PDE????-(Gt?)2. Our current goals are determining the structures of PDE6 in solution and bound to membranes, and the structure of the PDE????-(Gt?)2 assembled on the membrane surface.